The objectives of this study are to chart the development of sustained, endogenous attention in infants from 2 to 6 months of age, and to relate developmental trends in sustained attention to concurrent trends in heart rate and respiration. The specific aims are (1) to develop further a method for measuring sustained attention in infants, 2) to relate-resting measurements of heart rate, respiration, and respiratory sinus arrhythmia to visual, cardiac, and respiration responses that occur during visual attention, and 3) to determine how respiration and heart rate responses are related during visual attention. The techniques developed in these studies may provide a "model preparation" for studying pathological patterns of sustained attention found in hyperactive, autistic, and retarded children, and could lead to the early diagnosis of children with pathological patterns of attentional responsivity. There are four proposed experiments. For all of the studies, infants at 8, 14, 20 and 26 weeks of age (30 per age) will be tested. Heart rate and respiration parameters will be measured during a 5 min baseline and during the attention tasks. The studies will use an "interrupting stimulus" method to assess the level of sustained attention. In this paradigm, attention to a primary visual stimulus will be interrupted by a secondary task. The time it takes for the infant to be distracted from the primary task should be a measure of the attention resources dedicated to the task. In the first experiment, blinking lights in the periphery will be presented during active cognitive processing in order to interrupt attention to checkerboard patterns. In Experiment 2 the physiological responses following fixation termination of differing types will be studied. A single checked pattern will serve as the primary stimulus in a habituation paradigm in Experiment 3, with the blinking light as the secondary stimulus. The primary stimulus in Experiment 4 will be taken from several interesting pictures, and the secondary stimuli will be taken from the same set but shown in a different physical location to interrupt attention. It is predicted that 1) cardiac and respiratory variables recorded in the baseline will predict sustained attention responses, 2) sustained attention should increase with age, and 3) the increase in sustained attention will parallel developmental changes in baseline and concurrent heart rate and respiration parameters.